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ANTC23 W9.docx

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Joyce Parga

ANTC23 L9 Sperm Competition: - When sperm from 2 ore more males compete to fertilize the same egg(s) - Occurs whenever a female with more than 1 male during her fertile period Relative testes size in males indicates level of sperm competition - Larger relative testes size suggests more sperm competition - Larger testes = more sperm produced o i.e. Male vervet monkey Apes - Common chimps and bonobos have the largest relatives testes size and highest levels of sperm competition of any primate o Mating system? Polygynandry Orangutans - Smaller relative testes size than chimps, lower levels of sperm competition (but relative testes size is larger than in gorillas) - Mating system? Dispersed Gorillas - Small relative testes size, little to no sperm competition - Mating system? Often polygyny Gibbons and siamangs - Small relative testes size, likely very low levels of sperm competition - Mating system: Monogamy Testicle size and mating system - Body weight in bottom and testes axis in the Y. - All animals in black animals on top are MMMF where a lot of sperm competition is going on. - Owl monkey – they are monogamous, so they have small testes size relative to the body. Mating systems and sperm competition - Polygynandrous species tend to have the highest levels of sperm competition, polygynous species have lower levels, and monogamous species the least amount of sperm competition o Olive baboon o Proboscis monkey – polygynous. o Owl monkey - Sperm primates having dispersed mating systems can have relatively high levels of sperm competition o i.e. Aye aye o Some nocturnal prosimians Ring tailed lemur (Lemur catta) - Largest relative testes size of any prosimians - Indicates high levels of sperm competition - Polygynandrous mating system  those are the species with the largest testes size Sperm competition in humans? - Women do sometimes have sexual intercourse with more than one man when they are fertile - There is the potential for sperm competition - Testes are not large in relative to body weight (ratio is similar to orangutans) - Sperm competition was probably not a very strong selection pressure in human evolution Frequent ejaculation - Frequent ejaculation can lead to: o Decreased sperm numbers (low sperm count) o Decreased ejaculate volume Sperm competition - terms - Raffle principle: a male’s likelihood of fertilization success depends on the number of sperm he produces and inseminates o i.e. Male macaque - Sperm precedence: male fertilization success depends n mating order o Last male sperm precedence  i.e. zebra finches – if you have last sperm precedence, that will increase the chances of being the father o First male sperm precedence  i.e. Ground squirrel – you will have higher chance of being a father o Sperm precedence is usually not complete (not 100% assurance of paternity) Do primates show sperm precedence? - Earlier mating males have high fertilization success in this species (not necessarily 1 mating males) o Optimal insemination period is during early receptivity - i.e. mouse lemur (Microcebus murinus) - Some researchers argue that sperm precedence trends do not really exist for any mammal o |-------------------------
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